1. Field of the Invention
The present invention relates to a digital printer and an image data conversion method for the digital printer wherein image data obtained from a photographic image or the like is processed to be digital image data for reproducing the image as a hard copy.
2. Background Arts
In a conventional digital printer, an image signal is obtained through a photo-electric imaging device directly from a subject, or from a reflective original such as a photo-print or a transparent original such as a frame on a photographic film. The image signal is processed to obtain image control values. Based on the image control values, an image is reproduced and recorded on a photographic paper or another recording material. For example, JPA 2-157758 discloses determining a reference density point for each color component based on a highlight density and a shadow density in the original, and setting up a gradation conversion curve to assign these reference density values to given signal levels.
JPA 6-242521 discloses dividing a scene into several areas to detect a maximum value and a minimum value of each color from each area, determining a maximum reference value and a minimum reference value of each color from these maximum and minimum values of the divided areas. The maximum reference values of three colors, i.e. red (R), green (G) and blue (B), are reproduced as white, whereas the minimum reference values of three colors as black.
JPA 6-178113 discloses producing a histogram from image data, accumulating histograms of a plurality of images according to the type of recording media, and deriving gradation characteristics based on the image data of an original and the accumulated histogram data, to produce a conversion table for the image reproduction. This publication also discloses modifying the histogram to eliminate lopsided frequencies in some density levels, and selecting those images relating to one another among from a series of images, to utilize their data as image representative values for reproduction conversion curves. Specifically, JPA 6-178113 discloses to obtain weighted mean values of those three color density values which belong to a given accumulation degree, e.g. 50%, of a histogram made by accumulating data for each type of photographic films, and those three color density values which belong to a given accumulation degree, e.g. 50%, of a histogram made by accumulating data for each image, and produce such a conversion table that is adapted to make the weighted mean values equal to the former three color density values. Also, JPA 4-260274 and JPA6-253149 disclose pre-setting conversion tables for respective film types, and selecting one of those tables.
In almost all conventional digital image recording methods, highlight and shadow of the original are controlled to be reproduced as highlight and shadow in the reproduced image, as above described JPA 2-157758. This method is disadvantageous when the highlight and shadow of the original are not white and black. In a case where a scene is divided into a plurality of areas to determine maximum and minimum reference values of the three colors from maximum values and minimum values of the respective colors of the respective areas, like JPA 6-242521, dividing the scene can result in wrong reference values. If the maximum values and the minimum values of any area do not represent neutral gray, reproducing the maximum and minimum reference values as white and black would lower the quality or color balance of the reproduced image. To solve this problem, various methods for detecting gray points or neutral three color densities in a maximum density range have been suggested: manual designation of a gray point, reading image data from a color patch of the same type photographic film as the original, and so forth.
The method as disclosed in JPA 6-178113, wherein a histogram is made from image data and the histogram is accumulated in a memory provided for each type of recording media, is useful for those recording media like reversal films where exposure amounts on photography are substantially uniform and most images are properly exposed. However, for those recording media like negative films where color balance, gradation and other characteristic values vary depending upon exposure amounts on photography, the histogram varies according to the exposure amount even for the same photographic subject, so that it is difficult to exactly obtain a requisite film characteristic curve from simple accumulation of the histograms. Moreover, obtaining a weighted mean value is meaningless, because the three color density values obtained from the histogram of each image are required to be coincide with those obtained from the histogram of the corresponding film type, rather than their weighted mean values.
FIGS. 18A to 18C are density histograms for red (R-density) of images of the same subject recorded on a negative film, but respectively under-exposed, properly exposed and over-exposed ones. As seen from these diagrams, even if the images are photographed from the same subject on the same negative film, their histograms vary depending upon exposure conditions. Concerning originals on negative film, accumulation of their histograms does not result in a histogram for an average subject, but vainly equalizes and flattens the histograms.
The method of pre-setting conversion tables for respective film types and selecting one of those tables, such as disclosed in JPA 4-260274, is insufficient because the preset data is not optimized for each imaging device, and is not able to adapt to the variation in film characteristic curve.
Color balance of the negative film varies according to photographic exposure amount, film type, the light source used for photography, color fading of the photographic image. Moreover, according to the conventional color balance setting up method, reproduction conditions are determined for each image based on image data of each image, there can be color variance between the reproduced images.